Managing difficult urinary tract infections (Proceedings)
Urinary tract infection is the most common infectious disease of dogs, affecting as many as 14% of dogs over the course of their lifetime. The majority of these urinary tract infections (UTIs) are benign and respond readily to antimicrobial therapy. However, some patients with bacterial UTI do not respond to antimicrobial therapy and/or develop recurrent UTIs following withdrawal of antimicrobial therapy. Recurrence of clinical and/or laboratory signs of UTI may occur as a consequence of relapse (persistent infection), reinfection, or superinfection. Classifying recurrent UTI in this fashion is clinically useful because it provides guidance as to the possible cause for recurrent UTI (Table 1).
Relapses (persistent infections) are defined as recurrences caused by the same species and serologic strain of microorganism(s) within days to a few weeks of stopping antimicrobial therapy. In contrast, reinfections are recurrent infections caused by a pathogen different from that causing the previous infection. Reinfections are the most common form of recurrent UTI and typically occur more than a few weeks after stopping antimicrobial therapy. Superinfections are uncommon and are new infections which develop during the course of antimicrobial therapy. Urine cultures will be positive during or immediately after terminating therapy.
Confirming the Diagnosis of Bacterial UTI: Indications for Urine CulturesA presumptive diagnosis of UTI is often based on clinical signs and urinalysis findings. Detection of pyuria and bacteriuria by urine sediment evaluation is highly suggestive of bacterial UTI. However, bacteria are often difficult to detect on a urine sediment exam and white blood cells may be present for reasons other than infection. Additionally, many patients with a compromised immune system and/or polyuria, may not have cells detectable in the urine. Therefore, basing a diagnosis of UTI solely on clinical and urinalysis findings may result in both false positive and false negative diagnoses. (Table 2)
Although commonly treated with antimicrobial agents, lower urinary tract signs in young cats typically do not result from bacterial UTI. Recurrence of clinical signs is common in these cats, but recurrence rarely results from bacterial UTI except when urinary catheterization has been performed. Likewise, recurrent signs of lower urinary tract disease may occur in the absence of bacterial UTI in dogs with urolithiasis, neoplasia, or other urinary disorders.
If a presumptive UTI does not respond to antimicrobial therapy, a urine culture is indicated. Antimicrobial therapy should be withdrawn at least three days prior to collection of the urine sample for bacterial culture. Urine samples should ideally be cultured within 30 minutes after collection to optimize results. If immediate culture is not possible, urine may be stored refrigerated for up to six hours.